cql – a common query languagemike taylor cql – a common query language 1. what cql is 2....
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CQL – a Common Query Language Mike Taylor <[email protected]>
CQL – a Common Query Language
1.What CQL is
2.Motivation
3.Examples and explanation
4.Applications
5.Implementation
CQL – a Common Query Language Mike Taylor <[email protected]>
Chapter 1: What CQL is
● CQL is a query language:– For humans to type– For query forms to generate– For translating other languages into
● The only query language of SRW/SRU
● Also applicable in other contexts:– Z39.50 (instead of the Type-1 Query)– Query boxes for web searches
CQL – a Common Query Language Mike Taylor <[email protected]>
Chapter 2: Motivation
Most query languages fall into one of two camps:
● Complex and powerful, but cryptic and hard to learn– SQL, Prefix Query Format (PQF), XML Query
● Easy to learn and use, but lacking in power– Google, AltaVista, CCL
CQL aims to “make simple queries easy, and complexqueries possible” (to paraphrase Larry Wall, of Perl)
CQL – a Common Query Language Mike Taylor <[email protected]>
Learning curves for query languages
Power of query that can be expressed
Eff
ort
in learn
ing q
uery
lan
guag
e
SQL
CQL – a Common Query Language Mike Taylor <[email protected]>
Learning curves for query languages
Power of query that can be expressed
Eff
ort
in learn
ing q
uery
lan
guag
e
SQL
CQL – a Common Query Language Mike Taylor <[email protected]>
Learning curves for query languages
Power of query that can be expressed
Eff
ort
in learn
ing q
uery
lan
guag
e
SQL
CQL
CQL – a Common Query Language Mike Taylor <[email protected]>
Chapter 3: Examples and explanation
Important concepts
● Simple terms● Quoting● Booleans● Parentheses● Pattern matching● Word anchoring● Indexes● Prefixes● Context sets● Relations
Esoteric concepts
● Proximity● Relation modifiers● Boolean modifiers● Prefix mapping
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: simple terms
Here are some perfectly good CQL queries:
● fish
● Churchill
● dinosaur
● comp.sources.misc
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: quoting
Double-quote marks remove the special meanings ofspecial characters like space (which otherwise separatestokens) and of keywords such as “and” and “or”.
● "dinosaur"● "the complete dinosaur"● "ext–>u.generic"● "and"● "the \"nuxi\" problem"
(Backslash removes the special meaning of followingdouble-quote characters.)
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: booleans
The keywords “and” and “or” are boolean operators.The keyword “not” is an and-not binary operator.There is no unary negation operator. Case is notsignificant, so “AND” and “aNd” also work.
● dinosaur or bird● dinosaur not reptile● dinosaur and bird and reptile● dinosaur and bird or dinobird● dinosaur not theropod not ornithischian
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: boolean precedence
The “and”, “or” and “not” booleans all have equalprecedence and are evaluated left-to-right.
● dinosaur and bird or dinobirdMEANS
(dinosaur and bird) or dinobird
● dinosaur or bird and dinobirdMEANS
(dinosaur or bird) and dinobirdNOT
dinosaur or (bird and dinobird)
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: parentheses
Parentheses may be used to override the defaultleft-to-right parsing of boolean operators.
● dinosaur and (bird or dinobird)● dinosaur or (bird and dinobird)● (bird or dinosaur) and (feathers or scales)● "feathered dinosaur" and (yixian or jehol)● (((a and b) or (c not d) not (e or f and g)) and h not i) or j
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: pattern matching
There are two pattern-matching characters:* matches any number of characters? matches any single character
A preceding backslash removes their special meaning.
● dinosaur* – matches “dinosaurs”, “dinosauria”● *sauria – matches “dinosauria”, “carnosauria”● man?raptor – matches “maniraptor”, “manuraptor”● man?raptor* – matches the plurals of these● "the comp*saur" – matches “the complete dinosaur”● char\* – matches literal “char*”
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: word anchoring
A word beginning with “^” must occur at the start of itsfield. A word ending with “^” must occur at the end ofits field.
● dinosaur – matches “the complete dinosaur”● dinosaur^ – also matches● ^dinosaur – does not match● the – matches “the complete dinosaur”● ^the – also matches● the^ – does not match
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: indexes
A term of the form name=value is a query for the specifiedvalue occurring within the named index.
● title=Churchill – finds biographies of Churchill● author=Churchill – finds books written by him● title=dinosaur and author=farlow● title=(dinosaur and bird)● subject=(dinosaur* or pterosaur*)
Index names are case-insensitive, so “title” is the sameindex as “TITLE”, “Title” or “tiTLe”.
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: prefixes
The meaning of an index can be specified more fullyby a prefix indicating what context set it is from. Themeaning of “title” is different in cross-domain searching(Dublin Core), bibliographic searching (Bath Profile)and heraldry.
● dc.title="the complete dinosaur"● property.title=freehold● heraldry.title=(viscount or duke)● cql.serverChoice=fruit● cql.resultSet=YXJjaGJpc2hvcAp
Prefixes are case-insensitive.
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: context sets
A context set is a set of indexes that are related to aparticular area (plus some other more esoteric stuff thatyou can ignore).
For example, the Dublin Core context set containsindexes for searching against the fifteen DC elements:
title, creator, subject, description, publisher,contributor, date, type, format, identifier,source, language, relation, coverage, rights.
The context set prose must define their semantics.
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: some context sets
A few core sets created by the SRW editorial board:● CQL – for core indexes such as resultSet● DC – for metadata searching with Dublin Core● Rec – metadata about the record, not the resource● Net – network concepts such as hostname and port
Also, many application-specific sets:● Bath, Zthes, CCG, Music● Rel – deep voodoo for relevance matching● GILS is in development
Where do context sets come from?● You can just make them up! No-one can stop you!
CQL – a Common Query Language Mike Taylor <[email protected]>
A digression on the CQL context set
The CQL context set is special. It contains some “magic”indexes:
● cql.anywhere – searches in all the indexes available
● cql.serverChoice – allows the server to choose whateverindex or indexes are suitable
● cql.resultSetId – finds the records obtained in a previoussearch, e.g. for refinement by combining with otherquery terms.
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: relations
Usually “=” connects an index with its relation, but all theother obvious numeric relations are supported:
● Height = 13● numberOfWheels <= 3● numberOfPlates = 18● lengthOfFemur > 2.4● BioMass >= 100● NumberOfToes <> 3 (inequality)
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: special relations
The keywords “any” and “all” can be used as relations,indicating that any one of, or all of, the words specifiedin the term must be found in the index:
● author all "kernighan ritchie"– shorthand for
author=kernighan and author=ritchie
● author any "kernighan ritchie thompson"– shorthand for
author=kernighan or author=ritchie orauthor=thompson
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL features: esoterica
“You are not expected to understand this.”– comment in the Unix Version 7 source code.
The point is that new users are not required to understandthis, and may happily use CQL for many years – perhapsforever – without needing to.
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: proximity
The “prox” boolean, by default, requires its operandsto be next to each other, in either order:
● cervical prox vertebra– equivalent to
"cervical vertebra" or "vertebra cervical"
● (cervical or dorsal) prox vertebra– equivalent to
"cervical vertebra" or "dorsal vertebra" or"vertebra cervical" or "vertebra dorsal"
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: proximity II
Modifiers can generalise the semantics of proximity:
● cervical prox/distance<=5/ vertebrae– within five words of each other
● cervical prox/distance=0/unit=sentence vertebrae– within the same sentence
● cervical prox/distance>0/unit=paragraph vertebrae– in different paragraphs
● cervical prox/ordered vertebrae– in the specified order: exactly equivalent to
"cervical vertebra"
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: relation modifiers
Modifiers can refine the semantics of relations:
● title =/stem dig– finds “dig”, “digging”, “dug”, etc.
● title any/relevant "dinosaur bird reptile"– finds “sauropods”, “avian”, “crocodile”, “snake”, etc.
● author =/fuzzy tailor– finds “Mike Taylor”
● phoneNumber exact/fuzzy "020 8348 6768"– finds “020 8348 6769”
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: relation modifiers II
Relation modifiers can be overloaded to specify extrainformation about the term that the relation joins to theindex:
● createdDate >/isoDate "2004-03-12 09:45:00"– the term is in ISO 8601 format.
● Location within/geom.polygon "(12,46) (15,52)"– the term indicates a polygon of two points (i.e. a straight line) rather than the corners of a rectangle.
Modifiers can refine the semantics of boolean operators.We've already seen some examples of this in proximity.
● cervical prox/distance<=5/ vertebrae– within five words of each other
● cervical or/exclusive vertebrae– one or the other, but not both.
● "denenberg or/rel.mean "information retrieval"● "denenberg or/rel.sum "information retrieval"● "denenberg or/rel.max "information retrieval"
– average, total or maximum relevance of operands
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: boolean modifiers
So far, we have been free and easy with index prefixessuch as “dc”. But how do we know what they mean?
Why should “dc” mean Dublin Core rather than DeepCustard?● dc.custardDepth <= 20
Why should “bath” mean the Bath Profile for bibliographicsearching instead of plumbing supplies?● bath.capacityInGallons > 45
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: prefix mapping
Prefixes are just convenient, easy-to-type abbreviations.The real identifier of a context set is its URI.
For example, the Dublin Core context set isinfo:srw/cql-context-set/1/dc-v1.1
but we map that URI to a prefix for convenience.
This is exactly like XML namespaces: they are identifiedby URIs, but the URIs do not appear in the names ofelements or attributes: short prefixes are used instead.
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: prefix mapping II
In XML, a prefix is associated with a namespace using:● <element xmlns:prefix="http://example.org/xyz/">
In CQL, a prefix is associated with a namespace using:● >prefix=http://example.org/xyz/
and the rest of the query follows.
The following queries are exactly equivalent:● >dc=info:srw/cql-context-set/1/dc-v1.1 dc.title=fish● >yx=info:srw/cql-context-set/1/dc-v1.1 yx.title=fish
Most applications will have established default mappings.
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: prefix mapping III
It is possible to establish the context set from whichindexes with no explicit prefix are taken by omitting the“prefix=” part from the mapping:
● >http://example.org/heraldry/ title=baron and side=sinister
So the following queries are exactly equivalent:● >info:srw/cql-context-set/1/dc-v1.1 title=fish● >yx=info:srw/cql-context-set/1/dc-v1.1 yx.title=fish
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: prefix mapping IV
Finally ... Finally! :-)
Prefix mappings can be stacked up:
● >dc = info:srw/cql-context-set/1/dc-v1.1>bath=http://zing.z3950.org/cql/bath/2.0/>rec=info:srw/cql-context-set/2/rec-1.0
rec.created < 2004-10-09 anddc.title=ecology andbath.conferenceName=dinosaur
(Yes, this is all one query.)
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: prefix mapping V
Don't try this at home.
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: prefix mapping VI
CQL – a Common Query Language Mike Taylor <[email protected]>
Chapter 4: Applications
CQL has been deployed in many kinds of application:
● Google-like structureless searching● Simple metadata searching with the Dublin Core● Bath Profile for bibliographic data● Zthes profile for hierarchical thesaurus navigation● CCG for collectable card games● Music – musicalKey, arranger, duration, etc.● GILS (Global Information Locator Service)● ... your application goes here!
CQL – a Common Query Language Mike Taylor <[email protected]>
Chapter 5: Implementations
There are good-quality free CQL implementationsin several important languages:
● Java (Mike Taylor's CQL-Java package)● C/C++ (Adam Dickmeiss in Index Data's YAZ)● Python (Rob Sanderson in Cheshire)● Perl (Ed Summers' CQL::Parser module)● Visual Basic is in development (Thomas Habing)● ... your language goes here!
CQL – a Common Query Language Mike Taylor <[email protected]>
Conclusion: What to take home
● CQL makes easy queries easy and hard ones possible● You can use it well without learning the hard bits● It is used in SRW/SRU but also applicable elsewhere● It is extensible through context sets● Existing context sets support lots of applications● There are free implementations in several languages● Tutorial on-line at:
http://zing.z3950.org/cql/intro.html
CQL – a Common Query Language Mike Taylor <[email protected]>
CQL esoterica: relation modifiers II
Relation modifiers can be used to define essentially newrelations. Some hypothetical examples:
● location </geom.within "(12,46) (15,52)"– points within the specified rectangle
● task >/proj.prerequisite uiDesign– tasks that must be performed before the design of the user interface
● location =/geography.sameState "Las Vegas"– places in the same state as Las Vegas